If you don't remember the password to access the config page of the D-Link, a factory reset through the physical button is the only way indeed.
If you don't want to mess up your settings (you didn't save a backup either), and you want to keep the D-Link just as it is, add a wifi access point for the other party to connect to. In time though, you probably have to reset it anyway if you need to adjust something important.

OK, yes, could be done, but requires some basic knowledge/info on the router to do it.

1. How to get in to the router, IP address, router's admin user name/password
e.g. connected to the router by Ethernet (or, Wi-Fi), with browser giving IP address 192.168.0.1 or whatever alike is right for this router, found on the Eth/Wi-Fi Properties, giving -> admin/admin or whatever is right/defined instead these for this router

2. On the opened Settings Menu going to WLAN/Wi-Fi Security Settings changing/deleting the WEP/WPAx definitions/security key

Note: Some routers do not have the ability to recover your wireless password. If the following do not work, you will need to change the password altogether.

Method 1

Step 1: From a computer that is connected to the router (wired or wireless), Open your Internet browser and type the IP address of the router in the address bar. The default IP address is 192.168.0.1. At the login, enter the username (admin) and your password (default password is nothing).

Note: If you have forgotten this password, you will have no choice but to reset the router to factory defaults.

Reset Procedure: With the router powered on, Press and hold the reset button (on the back of the router) for 10 seconds and release. The lights on the front of the router should all flash and reboot.)-

Step 2: Click on the Setup Tab at the top and click wireless settings on the left side

Step 3: Click on Add device with WPS (Note: If this option is not available on your router, see below)

Step 4: Choose Manual

Your wireless network information should now be visible, including your wireless password.

Method 2

Step 1: Click on the Setup Tab at the top and click wireless settings on the left side

Step 2: Click on manual wireless network setup

Step 3: Scroll down the page to "Wireless Security Mode".

This is the section where you specify the wireless password.

Your password my be visible or it may be all dots. (If all dots you will need to re-enter a new password).

If specifying a new password, ensure to click save settings at the top of the page once complete.

Here’s the good news: because you’ve locked down your wireless network and router configuration settings, you’re one step ahead of anyone trying to access your network and its devices.

The bad news? Depending on which password you’ve forgotten – the Wi-Fi password that enables you to connect devices to your router, or the administrative password you use to log directly in to your router —you’re either in for a short visit to your router’s Web configuration screen or a total router reset.

The Forgotten Wi-Fi Password

I’ll start with the easy one: the forgotten Wi-Fi password. Once you’ve set and saved a Wi-Fi password on your laptop, after all, you won’t ever need to change it — though I do recommend you change all of your passwords a few times a year.

Resetting your D-Link® router’s Wi-Fi password is easy:

Type your router’s IP address —192.168.0.1— into your Web browser.
At the login screen, select “Admin” from the drop down menu and enter your admin password.
Once you access the Web configuration screen, click “Setup” in the top menu.
Select the “Wireless Settings” link on the left-hand side.
From there, click on the “Manual Wireless Connection Setup” button.
Scroll down to the field representing my “Pre-Shared Key”. If you don’t see it, select a security mode from the dropdown menu at the bottom of the screen first.
Type in a new password. (I use a hybrid WPA/WPA2 security mode.)

That’s it! Now for the more challenging of the two: the forgotten admin password.

The Forgotten Admin Password

If you forget your Web configuration password, you’re stuck: you have no way to change any settings on your router. Your only recourse is to use a paperclip or other pointy object to reset your router to its factory settings. With the
paperclip, hold down the tiny reset button on the back of your router for about five seconds. Your router lights will blink to confirm the factory reset. You can now use the router’s default password to log into its Web configuration screen (look in your router manual if you can’t remember what this is). Once you’re in, be sure to change this password to something unique. (To ensure a strong password, try Strong Password Generator.)

We all forget passwords. But with a little prep work, your router will be ready to rebound when you have a memory slip.

Thank you all. they were all good and all very useful. The guy I am helping with is not close and not that good at this so Dahra's makes it more easy for him to understand. This guy I am helping is not that savvy.

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